Field of the Invention
The present invention relates to a gas-passage change-over apparatus in which two gas lines are connected to one detector through a switching valve so that either of said two gas lines may be opened into said detector under the condition that the pressures at the ends of said two gas lines having different volumes are regulated by means of a pressure regulator.
Description of the Prior Art
The apparatus of the type described above has been used in a forced chemical luminescence type NO.sub.x -meter. The application to this NO.sub.x -meter is shown in FIG. 1. Referring now to FIG. 1, F designates a filter; P designates a sampling suction pump, and V designates a three-way electro-magnetic valve. A sample, for example, air, drawn in by means of the sampling suction pump P, is passed through a NO.sub.x -line 2, in which a converter 1 (an apparatus which changes NO.sub.2 into NO, concretely speaking, a cylindrical member housing reducers therein) is provided, and also passed through a NO (nitrogen monoxide)-line 3 without the converter; the lines are changed over by means of the electro-magnetic valve V, and the combined output of the lines 2 and 3 are then transferred to the detector 4 where the concentration of NO is determined. In cases where a sample is transferred to the detector 4 through the NO.sub.x -line 2, since not only NO contained in the sample but also NO obtained by the reduction of NO.sub.2, produce outputs, the concentration of NO.sub.x (.dbd.NO+NO.sub.2) can be determined while, in cases where said sample is transferred to the detector 4 through the NO-line 3, only NO contained in the sample produces an output, whereby the concentration of NO contained in the sample can be determined.
By the way, in analyzers such as a NO.sub.x -meter, the flow rate of a sample passing through the detector 4 must be controlled so as to be constant. To this end, also in such conventional apparatus the pressures at the ends A of the NO.sub.x -line 2 and the NO-line 3 are controlled by means of a pressure regulator Reg. However, since the NO.sub.x -line 2 is different from the NO-line 3 in volume, there is a problem that the pressures can not always be controlled so as to be constant. That is to say, since a converter is provided in the NO.sub.x -line 2, the volume of the NO.sub.x -line 2 is larger than that of the NO-line 3, so, in cases where the path of a sample is changed over from the NO.sub.x -line 2 to the NO-line 3, the converter 1 acts as a buffer, whereby the pressure of a pressure regulating portion (the end of line) is temporarily raised while, in cases where the path of the sample is changed over from the NO-line 3 to the NO.sub.x -line 2, contrary to that described above, the pressure of the pressure regulating portion is apt to be temporarily reduced. In general, since the regulating pressure of a pressure regulator in cases where the pressure is regulated from a lower pressure to a higher pressure is slightly different from that in cases where the pressure is regulated from a higher pressure to a lower pressure, the regulating pressure in the cases where the path of a sample is changed over from the NO.sub.x -line 2 to the NO-line 3 is different from the cases where the path of a sample is changed over from the NO-line 3 to the NO.sub.x -line 2. It was found from the measurement that the difference in regulating pressure reached about 2%. Such a difference in regulating pressure directly leads to an error of measurement. In addition, it leads to a serious problem in cases, where the efficiency of a converter is checked. Furthermore, referring to FIG. 1, element 5 designates a line for feeding the detector 4 with O.sub.3, and element 6 designates an ozonator, and element C.sub.ap designating a capillary.